Trichlorofluoromethane (known in the art as CFC-11) is currently available in commercial quantities and is used as a blowing agent for rigid urethane thermo-insulation foam. Also, 1,1,2-trichloro-1,2,2-trifluoroethane (known in art as CFC-113) is currently available in commercial quantities and is used as a solvent for cleaning integrated circuit boards. Currently, 1,1-dichloro-1-fluoroethane (known in the art as HCFC-141b) is considered to be a replacement for CFC-11 and CFC-113 because HCFC-141b does not deplete ozone in the stratosphere to the same extent as CFC-11 or CFC-113. Because the demand for HCFC-141b will increase dramatically in the future, commercially viable processes for the preparation of pure HCFC-141b are needed.
HCFC-141b may be produced by reacting vinylidene chloride with anhydrous hydrogen fluoride. The reaction is expressed by the following equation: EQU CH.sub.2 =CCl.sub.2 +HF.fwdarw.CH.sub.3 CFCl.sub.2
HCFC-141b may also be produced by reacting 1,1,1-trichlorothane with anhydrous hydrogen fluoride. The reaction is expressed by the following equation: EQU CH.sub.3 CCl.sub.3 +HF.fwdarw.CH.sub.3 CFCl.sub.2 +HCl
When vinylidene chloride is used, dichloroacetylene is present as an impurity in the feed stock and survives the reaction for the preparation of 1,1-dichloro-1-fluoroethane. Dichloroacetylene also passes normal purification processes such as distillation and ends up in the final product.
Regardless of whether 1,1-dichloro-1-fluoroethane is prepared by using vinylidene chloride or 1,1,1-trichloroethane with anhydrous hydrogen fluoride. vinylidene chloride is always present as an impurity in the final product. When vinylidene chloride is used as a starting material, residual amounts of unreacted vinylidene chloride are present in the final product. When 1,1,1-trichloroethane is used as a starting material, thermal decomposition of 1,1,1-trichloroethane results in the formation of small amounts of vinylidene chloride which are present in the final product. Because vinylidene chloride has a boiling point of 31.5.degree. C. and 1,1-dichloro-1-fluoroethane has a boiling point of 32.degree. C., removal of vinylidene chloride from 1,1-dichloro-1-fluoroethane is very difficult.
Dichloroacetylene is a very unstable and extremely toxic compound. Dichloroacetylene is mutagenic and has high carcinogenic potential. The Threshold Limit Value (TLV) for exposure to DCA is 0.1 part per million. Dichloroacetylene present in 1,1-dichloro-1-fluoroethane after distillation varies from 5 to 20 parts per million.
Vinylidene chloride is also a very toxic compound. Excessive exposure may cause damage to the kidneys, liver, or central nervous system. The TLV for exposure to vinylidene chloride is 5 parts per million. Vinylidene chloride present in 1,1-dichloro-1-fluoroethane after distillation varies from 500 to 1,200 parts per million. Although full scale toxicity studies for 1,1-dichloro-1-fluoroethane are currently incomplete, the Panel for Advancement of Fluorocarbon Test (PAFT II). which defines the purity of the 1,1-dichloro-1-fluoroethane product, has set current specifications on dichloroacetylene and vinylidene chloride at 1 part per million and 200 parts per million, respectively.
As such, a need exists for a process for removing or reducing the amounts of dichloroacetylene and vinylidene chloride in the 1,1-dichloro-1-fluoroethane product.
The purification of halogenated hydrocarbons has been the subject of many references. In general, these processes relate to the removal of reaction by-products which are difficult to remove by ordinary methods.
U.S. Pat. No. 2,879,228 teaches that hydrogen-containing impurities are removed by contact with alumina or silica gel at a temperature of 100.degree. to 250.degree. C. The impurities are adsorbed by the alumina or silica so as to leave a purified perfluorinated hydrocarbon.
Japanese Patent Application No. 48-103,502 teaches that sodium carbonate solutions are used to treat perchlorofluoroalkanes such as trichlorotrifluoroethane at 25.degree. to 60.degree. C. to remove hydrogen-containing halogenated hydrocarbons.
U.S. Pat. No. 3,696,156 teaches that alumina with 0.1 to 5% of an alkali metal or alkaline earth metal is used to remove unsaturated impurities to below 2 parts per million.
West German DE No. 3,017,531 teaches the purification of contaminated refrigerants such as trichlorotrifluoroethane by contact with alumina and an alkaline earth.
West German DE No. 3,311,751 teaches that a zeolite having a pore size of 0.4 to 1 nm is useful for removing halogens and inorganic halides from fluorochlorocarbons such as trichlorotrifluoroethane.
Japanese Patent No. 83-035,737 teaches regeneration of a zeolite used to purify and dry trichloroethane.
Russian Patent SU No. 743,985 teaches that chlorinated and fluorinated organic solvents are purified by passing the solvents in the vapor phase over activated charcoal in the presence of phosphorus pentoxide.
Commonly assigned U.S. Pat. No. 4,849,558 teaches that chlorofluorocarbon solvents such as 1,1,2-trichloro-1,2,2-trifluoroethane may be purified by removing sulfur dioxide by contact with alumina or zeolites.
Faced with the present problem, we considered various purification processes including selective adsorption. Selective adsorption requires an adequate balance between the polarity and pore size of the adsorbent and the dipole moments and molecular sizes of the adsorbates and solvent. The polarity and pore size of the adsorbent have to match the dipole moments and molecular sizes of dichloroacetylene and vinylidene chloride and to mismatch the dipole moment and molecular size of 1,1-dichloro-1-fluoroethane in order to absorb dichloroacetylene and vinylidene chloride selectively from 1,1-dichloro-1-fluoroethane. The dipole moment of dichloroacetylene is estimated to be 0.07 debye and the molecular size is 60 angstroms.sup.3. The dipole moment of vinylidene chloride is 1.34 debye and the molecular size is calculated to be 70 angstroms.sup.3. The dipole moment 1,1-dichloro-1-fluoroethane is 2.14 debye and the molecular size is 82 angstroms.sup.3.
We believed that molecular sieves which are made from a mixture of aluminum oxide and silicon dioxide and have a nominal pore size ranging from 3 to 5 angstroms would match the dipole moments and molecular sizes of dichloroacetylene and vinylidene chloride and mismatch 1,1-dichloro-1-fluoroethane so as to selectively absorb dichloroacetylene and vinylidene chloride from the 1,1-dichloro-1-fluoroethane. As set forth below in the Comparatives, molecular sieves were incapable of selectively absorbing dichloroacetylene and vinylidene chloride.
We then believed that silicalite which is made mainly from silica and has a pore size of about 5 to 6 angstroms would match the dipole moments and molecular sizes of dichloroacetylene and vinylidene chloride and mismatch 1,1-dichloro-1-fluoroethane so as to selectively absorb dichloroaetylene and vinylidene chloride from the 1,1-dichloro-1-fluoroethane. As set forth below in the Comparatives, silicalite was incapable of selectively absorbing dichloroacetylene and vinylidene chloride.